Redox flow batteries have characteristics such as (1) high safety, (2) a long charge-discharge cycle life, (3) ease of capacity increase, and (4) a capability of constantly monitoring the state of charge (SOC), and can be used in various applications. For example, redox flow batteries can be used for load leveling, for voltage sag compensation and emergency power sources, and for smoothing the output of natural energy, such as solar power generation or wind power generation that is being introduced on a massive scale.
In a redox flow battery, a positive electrode electrolyte and a negative electrode electrolyte are circulated and supplied to a battery cell which includes a positive electrode, a negative electrode, and a membrane disposed between the two electrodes, and charging and discharging are performed through a power converter (e.g., an AC-DC converter or the like). As the electrolytes, aqueous solutions containing metal ions (active materials) whose valence is changed by oxidation-reduction are used. Well-known examples include an iron (Fe2+/Fe3+)-chromium (Cr3+/Cr2+) based redox flow battery in which Fe ions are used as the positive electrode active material and Cr ions are used as the negative electrode active material, and a vanadium (V2+/V3+-V4+/V5+) based redox flow battery in which V ions are used as active materials for the positive electrode and the negative electrode.
In general, a redox flow battery requires pumps for circulating electrolytes to a battery cell. Accordingly, pump loss occurs. When the redox flow battery is operated always at a constant pump flow rate (electrolyte flow rate), pump loss may increase and battery efficiency may decrease in some cases. Therefore, in existing redox flow batteries, while adjusting the pump flow rate so as to correspond to the state of charge (which may also be referred to as “charging depth”) of the electrolytes, the electrolytes are supplied to the battery cell, thus reducing pump loss (for example, refer to PTL 1).
PTL 1 proposes a technique in which by reducing pump loss, battery efficiency is improved. Specifically, a cell resistance value is calculated from the measurement results of a cell terminal voltage, an open circuit voltage, and a load current, and on the basis of the cell resistance value, operation of pumps is controlled with an optimal electrolyte flow rate corresponding to the charging depth (open circuit voltage).